Andradite - Ca₃Fe³⁺₂(SiO₄)₃

Named from granatum (a pomegranate) for its resemblance to seeds of this fruit, Garnets are nesosilicates having the general formula X₃Y₂(SiO₄)₃. The X site is usually occupied by divalent cations (Ca²⁺, Mg²⁺, Fe²⁺) and the Y site by trivalent cations (Al³⁺, Fe³⁺, Cr³⁺) in an octahedral/tetrahedral framework with [SiO₄]^4- occupying the tetrahedra.

Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit. They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, sharp irregular pieces are formed.

Pyralspite garnets – aluminium in Y site

Pyralspite garnets show essentially complete solid solution among three end-member compositions, Pyrope-Almandine-Spessartine; minor solid solution with Ugrandite garnet may be present.

• Pyrope: Mg₃Al₂(SiO₄)₃
• Almandine: Fe₃Al₂(SiO₄)₃
• Spessartine: Mn₃Al₂(SiO₄)₃

Almandine: Named in 1546 by Georgius Agricola for Alabanda in Turkey, an ancient gem cutting center, presumably where almandine was fashioned into gemstones. Chemically, almandine is an iron-aluminium garnet with the formula Fe₃Al₂(SiO₄)₃; Almandine occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others.

Pyrope: From the Greek, pyropos, "fiery-eyed" in allusion to the red hue. Is red in color and chemically a magnesium aluminium silicate with the formula Mg₃Al₂(SiO₄)₃, though the magnesium can be replaced in part by calcium and ferrous iron. Pyrope is an indicator mineral for high-pressure rocks. The garnets from mantle-derived rocks, peridotites, and eclogites commonly contain a pyrope variety.

Spessartine: From Spessart Mountains, Bavaria, Germany is manganese aluminium garnet, Mn₃Al₂(SiO₄)₃. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites.

Optical properties of Pyralspite granet:
• Color: colourless to pink
• Form: the garnets are high in the crystalloblastic series and appear most commonly as euhedral crystals of dodecahedron or trapozohedron habit.
• Interference colors: isotropic
• Relief: high

Ugrandite group – calcium in X site

Extensive solid solution exist among the three and-members of the ugrandite garnet. Less solid solution exist between ugrandite and pyralspite garnet.

• Uvarovite: Ca₃Cr₂(SiO₄)₃
• Grossular: Ca₃Al₂(SiO₄)₃
• Andradite: Ca₃Fe₂(SiO₄)₃

Andradite: Named in 1868 by James Dwight Dana in honor of José Bonifácio de Andrada, Brazilian mineralogist, is a calcium-iron garnet, Ca₃Fe₂(SiO₄)₃, is of variable composition and may be red, yellow, brown, green or black. The recognized varieties are topazolite (yellow or green), demantoid (green) and melanite (black). Andradite is found both in deep-seated igneous rocks like syenite as well as serpentines, schists, and crystalline limestone. Demantoid has been called the "emerald of the Urals" from its occurrence there, and is one of the most prized of garnet varieties.

Grossular: Derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia, is a calcium-aluminium garnet with the formula Ca₃Al₂(SiO₄)₃, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. Grossular is found in contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite.

Uvarovite: Named after Count Sergey Semeonovich Uvarov, Russian statesman and scholar, President of the Academy of St Petersburg, is a calcium chromium garnet with the formula Ca₃Cr₂(SiO₄)₃. This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite, serpentinite, and kimberlites. It is found in crystalline marbles and schists in the Ural mountains of Russia and Outokumpu, Finland.

Optical properties of Ugrandite granet
• Color: colourless to pale yellow (grossular), pale to deep brown (Andradite) and emerald-green (ugrandite)
• Form: the garnets are high in the crystalloblastic series and appear most commonly as euhedral crystals of dodecahedron or trapozohedron habit.
• Interference colors: isotropic but sometime ugrandite garnet show a weak birefringence
• Relief: high

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Zoned andradite and aegirine in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)	Zoned andradite in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)	Zoned andradite in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)
Zoned andradite in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)	Zoned andradite in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)	Zoned andradite and aegirine in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)
Zoned andradite in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)	Andradite crystals in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)	Zoned Andradite in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)
Andradite crystals in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)	Andradite crystal in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)	Andradite crystals in a tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)